The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
At various stages of operation in a wellbore, such as after drilling, after completion, after an intervention operation, and so forth, debris may be generated in the wellbore. Examples of debris include sand particles or other particulates, and/or other solid debris. A well cleanout operation can be performed as a workover operation to remove such debris from the wellbore. Typically, a gelled water-based fluid is provided down a coiled tubing, with return fluid received in an annulus region outside the coiled tubing, where the return fluid contains suspended debris material.
Conventional cleanout operations can work well when a well reservoir is at a sufficiently high pressure. However, in certain wells, a well reservoir can have a relatively low pressure such that the well reservoir is unable to support a full column of water-based fluid. One technique for performing cleanout in an under-pressure well is to use a nitrogen-based foam as a service fluid. A foam has low density so that return fluid can be circulated to the earth surface even in a low-pressure well, and a foam has relatively good solid suspension properties. However, nitrogen-based foam is relatively expensive, and is not readily available in remote areas.
Another conventional technique of conducting well cleanout in an under-pressure well is to use concentric strings of coiled tubing, where two coiled tubings are concentrically provided and deployed into a well. Gelled water-based fluid (fluid in which a viscous material has been added to enhance viscosity of the fluid) can be provided down one conduit of the two-coiled tubing assembly and return fluid with suspended debris is circulated back to the earth surface through the other conduit of the two-coiled tubing assembly. However, running an assembly that includes two coiled tubings is associated with various issues, including increased weight, increased difficulty of transportation, and increased costs.